An opposed-piston, opposed-cylinder (OPOC) engine 10, as disclosed in U.S. Pat. No. 6,170,443, and incorporated herein in its entirety, is an asymmetrical configuration. Such an OPOC engine 10 is shown isometrically in FIG. 1. A first intake piston 12′ is the inner piston in one of the cylinders and a second intake piston 12 is the outer piston in the other cylinder. A first intake piston 12 and a first exhaust piston 14 reciprocate within a first cylinder; and a second intake piston 12′ and a second exhaust piston 14′ reciprocate with a second cylinder (cylinders not shown to facilitate viewing pistons). Exhaust piston 14 and intake piston 12′ couple to a journal (not visible) of crankshaft 20 via pushrods 16 (only one of which is visible). Intake piston 12 and exhaust piston 14′ couple to two journals (not visible) of crankshaft 20 via pullrods 18, with each of intake piston 12 and exhaust piston 14′ having two pullrods 18. Because the pullrods and pushrods sit adjacent to each other, a central axis 22′ of the left cylinder is parallel to, but offset from a central axis 22 of the right cylinder.
The movement of the intake pistons is displaced from the movement of the exhaust pistons such that the exhaust pistons precede the intake pistons in attaining their respective extreme positions by about 20 degrees. This is accomplished by asymmetrically orienting the eccentric journals on crankshaft 20 to which the pistons couple. By asymmetrically orientating the journals on crankshaft 20, the scavenging events are asymmetrically timed. The inertia forces, at a given engine speed, arising in the direction of reciprocation, X, is illustrated in FIG. 2 with the forces due to the outer pistons shown as dashed curve 70 and the forces due to the inner pistons shown as dash-dot-dot curve 72. The remaining inertia forces for all four pistons are shown as solid curve 74. If the timing of the pistons were not offset, there would be substantially no remaining imbalance. Even with the offset, though, the remaining imbalance is modest and much smaller than conventional engines, as will be discussed later in regards to FIG. 5A.
Although the balancing is nearly perfect for the engine of FIG. 1, such engine configuration does present a few disadvantages. In the process of optimizing the combustion chamber shape it is highly likely that the exhaust piston and the intake piston have different combustion chamber shapes. Furthermore, it is clear in FIG. 1, that the inner pistons and the outer pistons are distinct by their method of coupling to the crankshaft. Thus as inner and outer pistons are necessarily unique and intake and exhaust pistons are likely to be unique, engine 10 in FIG. 1 has four separate pistons: intake inner, intake outer, exhaust inner, and exhaust outer. To limit the number of individual parts for an engine assembly; it is desirable for the pistons to be arranged symmetrically, e.g., exhausts being inner pistons and intakes being outer pistons or vice versa. Also, the plumbing of the intake and exhaust is somewhat complicated in engine 10 (FIG. 1) due to an intake located between the two exhausts. Additionally, by having an inner intake piston and an inner exhaust piston coupling to the crankshaft adjacent to each other, the journals are split-pin type, i.e., they are not collinear with respect to each other, thereby requiring a small spacing between them making the engine wider and requiring additional strengthening measures.